A haboob is a unique type of dust and sandstorm characterized by a huge wall of dust at its leading edge. The term “haboob” is Arabic for “wind" or “to blow”. These storms only occur in a handful of places in the world. They were first observed on the southern edge of the Sahara desert but they can also be seen in the desert regions of Arizona, especially between Tucson and Phoenix, as well as regions in West Texas. Cities in the Sahara region generally experience several dozen haboobs a year but Arizona and Texas may only see one or two. While haboobs are most common between April and September they can occur during any month of the year. They are often mistaken as monsoon storms because they both happen during the summer months. However, the leading edge of monsoon storms is not a dust wall and the storms produce much more rain than haboob storms.
Interestingly, dust storms that are very similar to haboobs can be seen on Mars and are also thought to be seasonal.
Haboobs are generated from a relatively strong thunderstorm. Once this storm matures it starts to produce strong downdrafts of cold air near its leading edge. The air moves downward until it strikes the ground. Once reaching the ground the air moves along parallel to it, generating strong winds that pick up dust and sand. These winds help create a wall of dust at the edge of the storm. Once the haboob dust storm passes the thunderstorm generally follows, producing lightning, thunder, and rain.
Since I lived in Phoenix for over twenty years I have been lucky enough to experience many haboob storms. Most storms developed in the late afternoon of an otherwise uneventful summer day. The first indication that a storm is coming is a huge wall of dust that creeps over the landscape and slowly envelops the city. These walls can be over 3000 feet tall and are an impressive sight. Once the wall passes over dust and sand fill the air and winds can top 50 miles per hour. The debris in the air is often thick enough to block out the sun. During this time it is recommended that people stay inside, as the fast moving particles can hurt unprotected skin and cause serious injury to eyes. If the storm is really intense drivers are urged to pull their vehicles over to the side of the road and wait for the storm to pass. Car accidents are common during bad storms because of the reduced visibility. Erebr adds some good advice: "Drivers are recommended to turn off their lights before pulling off the road. Failure to do so can cause a moving car to follow you and crash into you when you stop." Fortunately, haboob storms normally last less than three hours.
http://www.weatheronline.co.uk/feature/wf260702.htm
http://www.cloudwall.com/wind/local/l17.htm
http://geography.asu.edu/aztc/monsoon.html